I was practising positioning divs by CSS and I found out margin of div being ignored. I cannot understand why margin-top of b2 is being ignored.
When I modified margin-bottom of b1, there was a space between two divs.
.b1 {
  width: 100px;
  height: 100px;
  border: 1px solid black;
  float: left;
}
.b2 {
  width: 100px;
  height: 100px;
  border: 1px solid black;
  clear: both;
  margin-top: 100px;
}<div class="b1"></div>
<div class="b2"></div> 
     
    